In product manufacture, determination and maintenance of the exact physical properties of the basic materials of which the product is made may be critical to the quality of the manufactured product. In the manufacture of coil springs, for example, the manner in which the material of which the springs are made deforms, both elastically and plastically, in response to loads applied in the manufacturing and in the use of the springs determines, in part, the useful life and performance of the spring and the ultimate product in which the spring becomes a part.
In the manufacture of such coil springs, which are typically made of metal wire, stress and strain properties of the basic wire material, when subjected to tensile forces must be known and maintained at certain design criteria. Such properties include the yield and tensile strengths of the wire, and the total and permanent tensile strains, or elongations under critical loads of the wire material of which the springs are made. Accordingly, regular periodic testing of the wire material during the product manufacturing process is essential to controlling quality in the product manufacturing operation.
Frequently, metal wire, metal sheet material and other types of such materials are supplied in coils. As a result, material removed from the coils often possesses a permanent or preset bend or curvature that reflects the permanent or plastic deformation that was imparted to the material when it is wound into a coil. This curvature has been an impediment to the accurate testing of such materials.
In the testing of coiled materials, for example in the elongation testing of high carbon steel wire used in the formation of certain types of springs such as often used for the springs for mattresses and box spring units, a sample of wire to be tested is removed from the coil and, before testing is carried out, is straightened. Then the straightened sample of wire is measured and some standard length is marked on the sample. Typically, the straightening and marking is carried out manually. The marked straightened length of wire is then placed in a tensile testing device and the sample is pulled to the breaking point. The distance between the two marks immediately prior to the breaking of the sample is taken as the total elongated length of the wire, and is due to both elastic and plastic deformation prior to breaking. This length, divided by the initial length, is the total elongation of the material. Then the two parts of the broken wire are placed into contact with each other at the failure point and the length between the two marks is measured. This length is the permanent or plastic deformation component of the total deformation. It is a number representing the fractional change in original length as calculated in accordance with ASTM standard A 370.
Several steps in the conventional elongation measurement process contribute to error in the measurement of elongation. The straightening of the wire, which is difficult to perfectly achieve, results in an error in the measurement of initial length. Imprecise straightening may result in some slack in the sample that adds to the initial length. Similarly, the marking procedure contributes to such error in establishing the initial length that is measured. The procedure of abutting the severed ends of the sample, following the test, to reconstruct the total deformed length prior to failure introduces error in the deformed length measurement. Overall, the measurement process is time consuming.
Various other methods have been devised to facilitate the testing of yield and tensile strengths and total and permanent elongation of various forms of materials. However, an easy to perform, quick and reliable method of performing periodic testing of coiled materials that have developed a permanent deformation or curvature when coiled remains a problem in manufacturing processes where the properties of such materials must be routinely monitored.
Accordingly, there remains a need for better, more accurate and more reliable methods for testing the material properties of coiled and other materials that develop a curvature for storage or transportation purposes.